Fluid bed boilers burning high sulfur coal are well known in the art. These boilers use classical bubbling bed technology whereby the fluid bed operates with superficial velocities in the range of 4 to 12 ft/sec and the bed is composed of particles with an average diameter of approximately 1000 microns. Coal is burned in the bubbling bed and limestone or dolomite sorbent is added to suppress the sulfur oxide emissions. The sorbent is added in particle sizes of 1000 to 3000 microns and the bed is composed largely of coal ash, spent sorbent, partially spent sorbent and partially burned fuel particles. The bubbling bed contains tubes within it to transfer heat to the steam. Tubes are also mounted above the bed in the freeboard to transfer heat from the hot combustion gases, thus cooling them. In operation, the bed elutriates fine particulates comprised of char, ash and partially spent sorbent. Many of these particles are captured by a recycle cyclone located downstream of the convective heat exchanger and these particles are returned to the bed in order to burn the fuel particles and allow unused sorbent to absorb more sulfur oxides. Very fine particles escape the recycle cyclone and are trapped in a filter system. The flow rate in the recycle loop is approximately equal to the total solids flow rate of the fuel and the sorbent fed into the combustor.
Conventional fluid bed boilers have several disadvantages. One disadvantage is that the combustion efficiency is low, approximately 97%, because small particles of unburned fuel escape to the combustion system. This problem would be vastly exacerbated if the boiler were to attempt to burn a low volatile content fuel such as petroleum coke which has 90% fixed carbon compared to 42% fixed carbon for coal. The second disadvantage is low sorbent utilization. A calcium to sulfur molar ratio of at least 3:1 must be maintained to produce sulfur oxide suppression of 90% to meet typical air pollution requirements. The reason for this is that the relatively large particles of sorbent only absorb sulfur oxides on their surface, leaving their interior material largely unused. A third disadvantage is that these boilers emit nitrogen oxides as a pollutant; the nitrogen oxides are generated from fuel-bound nitrogen. In many parts of the country the nitrogen oxide emissions do not exceed local limits but in some areas, such as California, they do. Moreover, a significant amount of combustion occurs in the freeboard volume above the fluidized bed. This results in fuel combustion in a zone where solids are in a very lean concentration and where there is only a very insignificant concentration of sorbent available to react with the sulfur oxide generated in that zone, as compared to the sorbent concentration within the bubbling bed itself. In addition, the continued heat extraction and associated cooling of gases in the freeboard zone do not provide the temperature conditions and residency time necessary to efficiently drive the reactions for sulfur oxide and nitrogen oxide suppression.
To improve combustion efficiency of conventional fluid bed boilers, Stewart et al. in U.S. Pat. No. 4,177,741 teaches the agglomeration of the recycled fines before reintroducing them into the bubbling bed. The agglomerated fines are thus prevented from being blown out of the bed and are thus encouraged to burn in the bed. Jones, U.S. Pat. No. 4,259,911 teaches agglomeration of coal fines plus recycled material before injection into the bed. To improve the utilization of sorbent, Jones U.S. Pat. No. 4,329,234 teaches the removal of a portion of the fluid bed and grinding the sorbent particles to 50 microns in diameter to fracture them, exposing new surface for additional sorption of sulfur oxides. The fractured particles are reintroduced into the bed by being agglomerated with the coal (fuel). All of these approaches are simple modifications of the classic bubbling bed boiler described earlier.
Reh et al. in German Pat. No. DE 3,023,480 describes a different approach to obtain good sorbent utilization in suppressing sulfur oxides from combustion gases. Reh et al. passes combustion gas through a fluidized bed of sorbent with particle size of 30 to 200 microns and a superficial velocity of 3 to 30 ft/second, producing an entrained bed with a particle density of 0.1 to 10 kg/cu m. The particulate entrained by the high gas velocity is removed by a recycle cyclone and returned to the bed, which is between 1300.degree. F. and 2000.degree. F. in temperature. The hourly recycle rate is approximately five times the bed weight. This approach achieves good sulfur oxide suppression by the use of fine particulate with large surface area and vigorous mixing. Reh however, does not teach combustion in the entrained bed of heat recovery with tubes from the entrained bed.
Reh in U.S. Pat. No. 4,111,158 described a fluid bed combustor based upon the principle of an entrained fluid bed which offers improvements in combustion efficiency, sulfur oxide suppression, nitrogen oxide control and turn-down. Whereas bubbling bed combustors operate with superficial velocities in the range of 4-12 ft/second and have a clearly defined upper surface, entrained bed combustors operate at superficial velocities of 15 to 45 ft/second and have no clearly defined upper surface but rather a gradation of particulate density from the bottom to the top of the combustor. The particulate is entrained with the gas flow in the reactor and separated from it by a recycle cyclone downstream of the reactor whereupon the particulate is reintroduced into the base of the reactor. Particle size ranges from 30 to 250 microns and the particle density of 10 to 40 kg/cu m in the upper portion of the reactor. Heat is not recovered from the particulate or gases in the reactor or recycle loop. Tubes in the reactor would be subject to high erosion and would not be effective in transferring heat because of the low particle density compared to that of a bubbling bed (500 kg/cu m). Heat is recovered by draining a portion of the bed from the base of the reactor and cooling it in a separate fluid bed heat exchanger optimized for that process. High combustion efficiency is obtained by completely burning small diameter fuel particles in the highly turbulent reactor and the hot recycle loop. Good sorbent usage is also obtained by using fine particulate and maintaining it at an effective temperature throughout the reactor and recycle loop. Limited nitrogen oxide control is obtained by progressively introducing combustion air along the length of the reactor. The disadvantage of the system is the need for the separate fluidized bed heat exchanger and large recycle cyclones.
The discussion immediately above related to a specific entrained fluid bed combustor which is described in more detail in Reh U.S. Pat. No. 4,111,158. In general, certain typical aspects of this type of combustor should be emphasized here. First, it should be noted that there is no clearly defined bed in the combustion zone of this type of combustor. Combustion occurs throughout the vessel including its freeboard and recycle cyclone. Heat is typically extracted throughout the freeboard to maintain constant temperature there. To this end, the outer walls defining the freeboard are cool (as a result of the presence of adjacent heat exchangers) and additional secondary combustion air is typically added to achieve complete fuel burnout. Thus, this approach lends itself to efficient suppression of sulfur oxide and nitrogen oxide.
Ammonia infection to suppress nitrogen oxides without a catalyst is taught by Lyon in U.S. Pat. No. 3,900,554. Lyon describes the basic gas phase reaction whereby ammonia selectively reduces nitrogen oxide in the presence of oxygen at 1742.degree. F. to 1832.degree. F. and predicts a suppression of 20% at an ammonia/nitrogen oxide molar ratio of 2. Lyon does not teach the benefits of good mixing, as in the recycle cyclone, which produced nitrogen oxide reductions of 95% at the same molar ratio of 2.